Optical sensing systems including optical interferometric sensors are being developed for a wide range of applications where they can offer benefits compared to conventional electrical sensor systems. These benefits include the ability to multiplex relatively large numbers of optical sensors on a single optical fiber to achieve a reduced form-factor for the optical fiber. Other benefits include immunity to electromagnetic interference, high sensitivity, and the ability to position the interrogation system relatively far away from the sensors. The interrogation system includes a light source to transmit light signals into the optical fiber, and a detection subsystem to detect light returned from the sensors.
Such optical sensing systems are often assumed to be immune to interference on the length of optical fiber between the interrogation system and the optical sensors (the downlead), even though the length of the optical fiber may be relatively long (several kilometers) and which may pass through terrain where the environment is not controlled. For a single mode system with zero birefringence, this assumption may be correct. However, in the vast majority of systems, this will not be the case for at least two reasons. First, the optical fiber itself will have some residual birefringence, although this amount may be small. Second, if an optical sensor with a reasonable form-factor is used, then the optical fiber will need to be coiled in order to reduce the space the optical fiber occupies, and this will induce birefringence into the optical fiber. In practical terms an optical sensor with birefringence has two optical paths with different lengths which can be addressed by changing the polarization launched into the optical sensor; as a result, the optical sensor output depends on the incoming polarization even if there is no change in the principal measurand (a characteristic being measured by an optical sensor). As the birefringence increases, so does the apparent sensitivity of the optical sensor to polarization.